[Oroboro]

There's an excellent bit of narration from the novel that highlights this perfectly, and is the other main reason Kirito figured out heathcliffs identity.

Spoiler for Light novel inner monologue:

Paraphrasing here, but Kirito describes Heathcliff looking cold and disinterested after the boss fight, while everyone else was totally worn out. Like he was looking down at rats in a cage. (As a reader, I figured it out the same second Kirito did with that. XD